Prior art blower motors are driven by a fixed driving voltage to rotate at a fixed number of revolutions to provide a desired amount of blast corresponding to the number of revolutions. The amount of intake air entering in the blower motor from the exterior air largely changes with the velocity of the car. A blower motor of this type is used in a car air conditioner disclosed in U.S. Pat. No. 4,482,009.
Changes in the amount of intake air cause that the blower motor driven at a fixed number of revolutions provides a variable total air supply because the total air supply is the sum of a fixed air supply responsive to the revolution of the blower motor and the variable intake air. A d.c. motor control circuit involving this problem is disclosed in U.S. Pat. No. 4,458,186.
A conventional type of an apparatus for controlling rotation of DC motor having the above-mentioned problems will be described in reference to FIG. 7.
A blower motor 2 for Dc driving to be connected to an anode of a battery 1 is energized and driven with a driving transistor 3. To the base of this transistor 3 is connected an output terminal of an operation amplifier 4, and to a non-inverted input terminal (+) of the operation amplifier 4 is supplied a sensing voltage Vd divided between the collector of the transistor 3 from a common connection point between the collector of the transistor 3 and the motor 2 to between the collector and the emitter of the transistor 3. To the inverted input terminal (-) of this operation amplifier 4 is supplied a reference voltage Va from a sliding contact terminal of a variable resistor 6 for use in setting a value. To one side of the fixed terminal arranged in this variable resistor 6 is applied a voltage through a resistor for limiting a variable range of the reference voltage Va from a battery 1. The other side of the fixed terminal arranged in this variable resistor 6 is connected to the ground. To a common connection point between a resistor for limiting a variable range of the reference voltage Va and one side of the fixed terminal arranged at the variable resistor 6 is connected a cathode of Zener diode 7 for preventing a variation amount of power supply voltage from influencing against the reference voltage Va, and its anode is connected to the ground. A cathode of the battery 1 not shown is connected to the ground and a power supply voltage is supplied from this battery 1 to an anode power supply terminal (not shown) of the operation amplifier 4 for performing a differential amplification. A cathode supplying terminal of a power supply (not shown) of this operation amplifier is connected to the ground. This operation amplifier 4 is constructed such that circuit elements for setting a differential amplifying operation (not shown) to the output terminal and the input terminal are provided.
Now, a sensing voltage Vd which is also a potential at a common connection point between the motor 2 and the transistor 3 applied to both ends of the motor 2 and outputted in correspondence with a load condition of the motor 2 has a relation of values indicated in the following equation (that is, a voltage value in which the power supply voltage Vcc is subtracted by a driving voltage applied to both ends of the motor 2) among the power supply voltage Vcc supplied by the battery 1, an electro-mechanical load resistance value Rm of the motor 2 and a driving current I energized to the motor 2. EQU Vd=Vcc-Rm.I
Therefore, in case that the load resistance Rm of the bower motor 2 is constant, the sensing voltage Vd which is a desired driving voltage is applied to the blower motor 2, so that the number of revolution of the blower motor 2 becomes constant and then a specified amount of air is supplied from the automobile air conditioner into the automobile.
Practically, an air pressure fed from an external part is varied in response to a variation of an automobile speed and the like and an amount of intake air for the automobile air conditioner is increased or decreased, so that the load resistance Rm of the blower motor 2 is decreased or increased and as apparent from the above equation, the sensing voltage Vd is decreased or increased. Then, in case that the sensing voltage Vd is decreased, (a condition in which the load resistance Rm of the motor 2 is increased), an output voltage Vo of the operation amplifier 4 is also apt to increase, so that the driving current I is increased and the sensing voltage Vd which is a potential at the common connection point of the transistor 3 and the motor 2 is increased. In turn, in case that the sensing voltage Vd is increased (a condition in which the load resistance Rm of the motor 2 is decreased), the output voltage Vo of the operation amplifier 4 is also apt to decrease, so that the driving current I is decreased and the sensing voltage Vd which is a potential at the common connection point between the transistor 3 and the motor 2 is decreased. In this way, since a desired driving voltage based on the set voltage Va is always applied to both terminals of the blower motor 2, the blower motor 2 assures a constant number of revolution to supply air into the automobile.